1. Technical Field
The present invention relates to a switching power-supply unit for pulse width modulation (PWM) systems, frequency-control or phase-control systems using resonance and the like.
2. Prior Art
Conventionally, a widely used switching power-supply unit switches a direct current generated from a rectified and smoothed commercial alternating current at a high frequency such as, say, 100 kHz and effectively converts the current to a desired voltage using a transformer.
This switching power-supply unit uses output voltage control systems such as a PWM control system which controls a duty ratio of switching pulses according to output voltage changes, a frequency-control or phase-control system using resonance and the like which controls switching pulse frequencies or phases.
A PWM-controlled switching power-supply unit provides control to narrow an ON duty for stabilizing an output voltage when a load power becomes extremely small during a wait state in which no current is loaded. It is impossible to zero an ON duty for maintaining constant oscillation A constant ON duty is ensured by providing a dummy resistor.
Accordingly, in the PWM-controlled switching power-supply unit, a power needed for the dummy resistor prevents power consumption from being minimized.
When no dummy resistor is used, increasing an output voltage zeros the ON duty to stop the oscillation for stabilizing the output voltage. As the oscillation stops and the output voltage decreases repeatedly, an intermittent oscillation occurs at 5 to 10 kHz. This intermittent oscillation decreases a switching frequency to a sonic frequency band, and may generate a grating noise at an inconsistent cycle.
A resonance-control switching power-supply unit provides control to increase an oscillation frequency for stabilizing an output voltage when a load power becomes extremely small during a wait state in which no current is loaded. However, increasing an oscillation frequency also increases losses in transformers and switching elements, thus preventing power consumption from being minimized.
The present invention has been made in consideration of the foregoing with respect to a conventional switching power-supply unit. It is therefore an object of the present invention to provide a switching power-supply unit which minimizes power consumption during a wait state without generating a grating noise.
The above object can be attained by providing a switching power-supply unit having a rectifier circuit for converting an alternating current from an alternating power supply to a direct current, a switching element for switching a current rectified by this rectifier circuit, and a primary coil for supplying a current to be switched by this switching element, including:
a transformer having a secondary coil for inducing power corresponding to current supplied to this primary coil;
DC output means for rectifying and smoothing a power induced at this transformer""s secondary coil to output a secondary direct current power;
switching control means for controlling a switching operation of said switching element so as to output a specified voltage from this DC output means; and
control means for controlling operations of said switching control means,
wherein said control means, in the normal operation mode, continuously operates said switching control means so as to operate said switching element at a basic oscillation frequency, wherein said control means, in the standby mode, intermittently operates said switching control means so as to turn on said switching element at a given repetition interval and for a given time period, wherein that repetition interval is shorter than a time for decreasing an output voltage from said DC output means down to a guaranteed load operating voltage due to power consumption of the load and said DC output means in the standby mode and is longer than a time for entering a sonic frequency band, and wherein that time period is longer than a time for stabilizing an output from said DC output means and is longer than a time for entering a sonic frequency band.
A switching power-supply unit according to the present invention can minimize power consumption during a wait state without generating a grating noise by continuously operating the switching control means in the normal operation mode so as to operate the switching element at a basic oscillation frequency, or by intermittently operating the switching control means in the standby mode so as to turn on the switching element at a given repetition interval and for a given time period, wherein that repetition interval is shorter than a time for decreasing an output voltage from the DC output means down to a guaranteed load operating voltage due to power consumption of the load and the DC output means in the standby mode and is longer than a time for entering a sonic frequency band, and wherein that time period is longer than a time for stabilizing an output from the DC output means and is longer than a time for entering a sonic frequency band.